U.S. DEPARTMENT OF THE INTERIOR TO ACCOMPANY MAP I-2390 U.S. GEOLOGICAL SURVEY

GEOLOGIC MAP OF THE MONTARA MOUNTAIN AND SAN MATEO 7-1/2 1 QUADRANGLES, SAN MATEO COUNTY, CALIFORNIA By Earl H. Pampeyan

INTRODUCTION Darrow (1963), Mclaughlin (1969), Jack (1969), Pampeyan The Montara Mountain and San Mateo quadrangles (1975; 1981a, b), Helley and others (1979), Morgan span the San Francisco Peninsula and Santa Cruz (1981a), and numerous researchers studying topical Mountains in northern San Mateo County. The area stratigraphic, geologic, and geotechnical problems. In covered by this geologic map extends from precipi­ addition, a series of maps prepared in cooperation with tous cliffs and flat marine terraces along the Pacific the county of San Mateo and based on the geologic map Coast, eastward across rugged topography, to gently of San Mateo County (Brabb and Pampeyan, 1983) include sloping alluvial plains and tidal and marsh lands of San the present map area. The interested reader is referred Francisco Bay. The rugged topography is cut by several to Brabb and others (1982), Brabb (1983), Wieczorek northwest-trending linear trenches, the two most and others (1985), Thomson and Evernden (1986), Brabb prominent of which are along the San Andreas and and Olson (1986), Youd and Perkins (1987), Perkins (1987), Pilarcitos Faults. The map and adjoining areas are and Mark and Newman (1988) for information pertain­ divided into four structural blocks juxtaposed along major ing to history, slope stability, seismic shaking, liquifaction faults (fig. 1), adopting the scheme of Nilsen and Brabb potential, and faulting and seismicity in the county. For (1979): the San Francisco Bay block lying east of the information on physical and engineering properties of San Andreas Fault; the Pilarcitos block lying between the map units the reader is referred to Wentworth and the San Andreas and Pilarcitos Faults; the La Honda others (1985). block lying between the Pilarcitos and Seal Cove-San Field work for this geologic map was done in 1970- Gregorio Faults; and the Pigeon Point block lying west 1973 and 1980 (Pampeyan, 1981a, b). Some minor of the Seal Cove-San Gregorio Fault. In this area the additions and revisions were made in 1982, but numer­ Pilarcitos Fault marks the boundary between Salinian ous changes caused by post-1980 grading operations in granitic basement of the La Honda and Pigeon Point connection with urban development are not shown. blocks and Franciscan Complex basement of the Pilarcitos and San Francisco Bay blocks, each of which is overlain by Cenozoic coarse- to fine-grained clastic sedimen­ STRATIGRAPHY tary units. On the geologic map the San Andreas, PRE-CRETACEOUS ROCKS Pilarcitos, and Seal Cove Faults are indicated by a single The oldest(?) rocks present in the map area are small line representing the most prominent and most recently pendants of metasedimentary rock (unit pKm) in the granitic active trace in the respective zones of faulting. rock of Montara Mountain (unit Km). One pendant near Most of the bayside flatland area has been covered the south-central edge of the map area, is marble; the with residential and commercial developments so that only other pendants, near the south edge of the map area vestiges of historic alluvial deposits and tidal and marsh in El Granada, consist of three lenses of schist and hornfels. lands remain. San Andreas and Crystal Springs Lakes No other pendants were seen in the granitic terrane, are rnan-made reservoirs in San Andreas and Crystal Springs although others may be present beneath the mantle of Valleys, and Pilarcitos Lake is a man-made reservoir on surficial deposits. Lawson (1914) correlated the marble Pilarcitos Creek. These lakes and the surrounding watershed with the Paleozoic(?) Gabilan Limestone in Monterey lands are part of the water supply system for the city County, a dubious correlation at best. Lacking diagnostic and county of San Francisco. The watershed, also a state data, the pendants are considered to be remnants of a fish and game refuge, is largely undeveloped except for pre-Cretaceous sedimentary unit. facilities of the San Francisco Water Department. The coastal side of the San Francisco Peninsula contains open­ JURASSIC AND CRETACEOUS ROCKS space land as well as pockets of dense urbanization, but Sedimentary and igneous rocks of the Franciscan development there has been limited by poor access, Complex, most of which are metamorphosed to some inadequate utility facilities, and California Coastal Zone degree, are the next oldest units in the map area, and restrictions. Municipalities in the map area include Pacifica, they form the basement of the Pilarcitos and San Fran­ San Bruno, Millbrae, Burlingame, Hillsborough, San Mateo, cisco Bay blocks. They are considered to be Jurassic Foster City, Belmont, San Carlos, Redwood City, and and Cretaceous in age though no Jurassic index fossils unincorporated districts of Montara, Moss Beach, and are known from this area. In the Pilarcitos block the El Granada (fig. 2). dominant Franciscan units are greenstone (unit fg) and Geologic maps of all or parts of the present map sandstone (graywacke) (unit fs), with a band of sheared area have been prepared by Lawson (1914), Glen (1959), rock (unit fsr) along the Pilarcitos Fault. Limestone.

1 \ .. 37"45' ~ 7~ C). .U">,....., "-b \ \ ~ \ 7.L \ \ \ San Francisco' ~ay block \,Redwood Point \Quadrangle

La Honda block

0 5 10 MILES

Figure 1. Map showing relation of study area (horizontal lines) to major tectonic bound­ aries of the northern Santa Cruz Mountains. Diagonal-lined area is Palo Alto and part of the Redwood Point quadrangles (Pampeyan, 1993).

2 122°30' 122°22'30"

37" 37'30"

37° 30'

Figure 2. Geographic map of study area and vicinity. serpentinite, conglomerate, chert, and glaucophane schist, volcanic rock to the south in the adjoining Santa Cruz in decreasing order of abundance, also are present. 30' quadrangle (Haehl and Arnold, 1904; Branner and Greenstone in the northern part of the map area is largely others, 1909). The basaltic rocks that I saw on Belmont agglomerate, flow-breccia, and tuff; in the southern part Hill had no lithologic resemblance to Miocene basalt to of the map area, greenstone consists of flows with or the south (Pampeyan, 1970, 1993) or elsewhere in San without pillow structure. The greenstone unit appears Mateo County (Brabb and Pampeyan, 1983), they were to be interbedded with graywacke, though the structure Franciscan greenstone. may be more complex than described by Lawson (1914). Rhythmically bedded, grayish-red to dark-brown In the San Francisco Bay block, sheared rock (unit fsr) radiolarian chert interlayered with graywacke and greenstone of the Franciscan Complex is the dominant unit. It contains is well exposed in San Carlos, Belmont, central San Mateo, tectonic inclusions of greenstone, graywacke, glaucophane and at Coyote Point. There are other moderate-size schist, and chert scattered about in random fashion. Along exposures of chert, most of which are tectonic inclusions the east edge of the sheared-rock unit, coherent bodies in sheared rock. Many of the remaining smaller bodies of chert, graywacke, and conglomerate are present; along are yellowish-orange, grayish-green, or white, and some its southwest edge the sheared rock unit is covered by of them are recrystallized and have fractures and voids serpentinite. up to a few inches across lined with quartz crystals. Sandstone (unit fs} of the Franciscan Complex, Radiolarians from shale interbeds at the site of ANGELO commonly referred to as graywacke, is widespread in the were described by Riedel and Schlocker (1956) as hav­ Pilarcitos block where it appears to be interbedded with ing similarities with some Jurassic and Cretaceous spe­ predominantly pyroclastic greenstones. In the San Francisco cies, and a Jurassic or Cretaceous age was postulated Bay block the largest bodies of coherent graywacke border for this fauna by Bailey and others (1964). the flatlands between San Carlos and San Mateo. Tectonic Limestone bodies extend diagonally southeast across inclusions of graywacke present in the sheared rock unit the map area and, with one exception, are all in the are relatively small and not easily recognized as such in Pilarcitos block The largest body was at Rockaway Beach natural exposures. One unique tectonic inclusion of at the mouth of Calera Valley, which Lawson (1914) defined graywacke exposed during construction of Highway 92 as the type Calera Limestone Member of the Cahil near the College of San Mateo contained seams of Sandstone. More recent reports have revised the Calera anthracitic coal 2 in. thick (G.O. Gates, oral commun., as a separate formation (Brabb and others, 1982). Little 1970). of the type Calera Limestone outcrop remains today owing Northeast of Belmont Hill, between El Camino Real to quarrying operations in the area. Much of the re­ and Highway 101, a small hill underlain by conglomer­ maining limestone is interbedded with graywacke and ate, chert, and greenstone was the site of U.S. Coast greenstone, and some is surrounded by sheared rock (unit Survey (1853b, 1857b) triangulation station ANGELO. fsr). Along the west edge of Lower Crystal Springs Lawson (1914) believed that this conglomerate, and another Reservoir, many of the small bodies of limestone are small patch of conglomerate a half mile southwest of elongate parallel to the San Andreas Fault; these bod­ Belmont Hill, probably represented the Knoxville For­ ies appear to be fault slivers, but outcrops typically do mation. The small hill, later referred to as Quarry Hill, not expose details of the contact with enclosing rocks. was removed by grading about 1951 but exposures of According to W.V. Sliter (oral commun., 1991), fora­ pebble to cobble conglomerate (unit fcg} remain and minifers from the type locality of the Calera indicate an undoubtedly are part of the Franciscan Complex. The Aptian to Cenomanian (mid-Cretaceous) age, similar to small patch of conglomerate to the southwest consists limestone in the Permanente quarry 30 mi southeast across of well-rounded chert pebbles in a silty to clayey matrix the San Andreas Fault (Tarduno and others, 1991), except and resembles chert-pebble conglomerate lenses and beds that at the quarry the section extends upward into the in Franciscan graywackes on Belmont Hill, exposures of Turonian Stage. Rare megafossils from Franciscan rocks which were too poorly exposed to determine their total in San Francisco (north of map area) and at New Almaden extent. Cobble conglomerate similar to that at ANGELO (south of map area) also indicate a mid-Cretaceous age is present along the north edge of San Pedro Valley in (Bailey and others, 1964). A lone small body of pale­ the Linda Mar district of Pacifica and, though poorly red limestone seen in the San Francisco Bay block near exposed, extends for about a mile either as a single thin the head of Mills Creek in San Mateo, believed to be block in sheared rock or as five more or less aligned in its natural setting, lithologically resembles the infor­ northwest-trending detached elongate blocks. The con­ mally named Laytonville limestone of Alvarez and oth­ glomerate at ANGELO is composed of weathered cobble­ ers (1980) (equivalent to the Laytonville-type limestone size clasts in a graywacke matrix, and at Linda Mar the of Bailey and others, 1964) of the Franciscan Complex conglomerate is lithologically similar except that locally of northern California, but the correlation is not well enough it contains some small-boulder-size clasts. established to extend the unit name to the area of this Altered mafic volcanic rock is widespread in the map. Pilarcitos block and scattered about in the San Francisco Tectonic inclusions of metamorphic rock typically Bay block. This rock type, referred to as greenstone, surrounded by sheared rock or serpentinite are present is composed mostly of coarse pyroclastic ejecta but also in the San Francisco Bay block, but are not restricted contains small intrusive plugs and flows locally showing to the sheared rock and serpentinite units. The inclu­ pillow structures. Lawson's (1914) map shows basaltic sions represent the blueschist metamorphic facies and rock on Belmont Hill which he correlated with Miocene have schistose to gneissic textures. Some inclusions are

4 as large as 30 ft across but most are much smaller. In­ Springs Dam, and evidence of its presence exists even clusions of blueschist also are present in sheared rock after urban development of the area. Other small north of San Pedro Valley, but they are too small to show occurrences of silica-carbonate rock are present in the on the map. map area; all but two, one 0.8 mi west-northwest and Sheared rock (unit fsr), or melange (Hsu, 1968), is the other 1. 9 mi southeast of Crystal Springs Dam, are the predominant Franciscan unit of the San Francisco too small to show on the map. In the California Coast Bay block, and it also underlies a significant part of the Ranges, mercury deposits commonly occur in silica-car­ Pilarcitos block. Excellent outcrops of this unit were exposed bonate rock, and stratigraphic relations indicate that the during construction of Highway 92 between San Mateo mineralization that produced the silica-carbonate rock and and Interstate 280. The unit consists of blocks or tectonic associated mercury deposits is related to middle Tertiary inclusions of graywacke, greenstone, chert, glaucophane (late Miocene or younger) volcanism (Bailey and others, schist, and limestone in a sheared mudstone matrix. Shear 1964). No metallic sulfide minerals were seen in the foliation in this unit typically is horizontal or shallow­ silica-carbonate-rock. dipping in contrast to vertical shear foliation in the San On the geologic map two areas are shown as meta­ Andreas Fault Zone. North of San Pedro Valley and in igneous and metasedimentary rocks, undivided (unit fu) places northwest of Buena Vista School (between Parrott of the Franciscan Complex. One is on Buri Buri Ridge, Drive and Polhemus Road), this unit is deeply weathered where landscaping and grading for the Crystal Springs and erodes to badlands topography. Cowan (1985) Golf Course have obscured bedrock relations, and the proposed a four-fold classification scheme for melanges: other is on the southwest slope of Cahill Ridge, where I, disrupted sandstone and mudstone strata; II, disrupted outcrops of Franciscan rocks are inaccessible or too poorly thin beds of chert, greenstone, and minor sandstone preserved to assign to a specific lithologic unit. The geology originally interbedded with mudstone; Ill, block-in-matrix of these areas may be as straightforward as shown by mudstone chaos, inclusions of diverse shapes, sizes, and Lawson (1914) but present-day exposures indicate a more compositions in a mudstone matrix; and IV, lenticular complex distribution of units. inclusions of diverse types bounded by an anastomos­ In his report on the San Francisco Bay region Lawson ing network of faults. Sheared rock of the San Fran­ (1914) divided the Franciscan rocks of this area into cisco Bay block matches the description of type Ill melange formations, for example, the now abandoned Cahil Sand­ and those of the Pilarcitos block appear to grade from stone and Sausalito Chert. His Cahil Sandstone con­ type I to III. The contact between sheared rock and sisted of graywacke and included the Calera Limestone uninterrupted Franciscan units in the San Francisco Bay Member along with small lenses of chert and limestone. block is at least locally interpreted as a high-angle fault. He correlated the large chert bodies around Belmont with Exposures of this contact in urbanized areas of the map his Sausalito Chert in Marin County and relegated smaller area are poor, but they plot as curvilinear boundaries chert bodies and lenses to the Cahil Sandstone. He made which may indicate low-angle faults. In the Pilarcitos no mention, however, of the random distribution of chert, block the north edge of the sheared rock unit is irregu­ greenstone, limestone, and metamorphic rock bodies in lar-possibly the result of repetition along high-angle faults a comminuted matrix of shale and graywacke and mapped parallel to the Pilarcitos Fault. Origin of melanges and those areas as the Cahil Sandstone. their relation to the west edge of the North American Plate is discussed in detail by Fox (1983). CRETACEOUS ROCKS Serpentinite is widespread in the San Francisco Bay Basement rock of the La Honda and Pigeon Point block where it occurs predominantly in a large flat-ly­ blocks is granitic, but is so pervasively fractured that sound ing to gently west dipping sheet overlying sheared rock hand specimens representative of the unit are difficult and also as near-vertical lenses and pods. The sheet dips to obtain. Narrow aplite and pegmatite dikes are com­ down to the San Andreas Fault Zone where it terminates, mon, and in a few places buff-weathering rhyolite dikes and the lenses generally parallel the San Andreas Fault. as wide as 2 ft are present. The granitic rock of Montara Serpentinite is less abundant in the Pilarcitos block where Mountain has been variously called granite (Lawson, 1893, it occurs mainly as lenses along faults and locally as pods p. 150; Law5on, 1895, p. 8), quartz diorite (Branner in sheared rock. Lawson (1895, 1914) considered the and others, 1909; Lawson, 1914, p. 4; Curtis and others, lenticular bodies to be dikes and the large serpentinite 1958, p. 9; Compton, 1966, p. 285; Ross, 1972, p. sheet to be an unroofed laccolith, but Fairbanks (1897) 10), granodiorite (Darrow, 1963, p. 9-11), and tonalite argued that the sheet was a sill instead of a laccolith. (Ross, 1984, figs. 4, 6), the differences largely reflect­ The steeply dipping lenticular bodies may be remnants ing the nature of study and the choice of classification of serpentinized ultramafic fault-controlled dikes, but some scheme. Most workers agree, however, that the ratio are randomly oriented tectonic inclusions in sheared rock. of potassium feldspar to plagioclase varies from place The large sheet of serpentinite, rather than being an intrusive to place within this pluton. Modal analysis of 22 me­ body, may have flowed from a fissure onto an erosion gascopically identical specimens by Ross (1972) and 25 surface as described by Carlson (1984) elsewhere in the specimens from this study, using the classification scheme Coast Ranges and subsequently been truncated by the of Streckeisen (1973), showed the composition to range San Andreas Fault. from tonalite to granite (fig. 3). With the exception of In a few places the serpentinite has been hydrother­ dike rocks and one locality 1 mi east of South Peak where mally altered to silica-carbonate rock (unit sc}. Lawson coarsely crystalline felsic rock contains some pyrope(?) (1914) noted one large body a half mile east of Crystal garnet, all exposures of this pluton contain an abundance

5 of hornblende and biotite. Foliation in the pluton is steep The sequence is folded and faulted and the base of the and is indicated by orientation of mafic minerals, tabu­ lower part is not exposed. The lowest exposures crop lar dioritic inclusions, and dikes; strike of the foliation out between Point San Pedro and the north point of Shelter ranges from northeast to northwest. Potassium-argon Cove and consist mostly of alternating turbitite sandstone age determinations on two samples of granitic rock (Cali­ and shale beds. The basal boulder conglomerate of the fornia Division of Mines and Geology, 1965; Curtis and upper part (unit Tsu) is channeled into the lower part others, 1958; Compton, 1966), recalculated using 1977 and is well exposed along the east-west part of High­ constants (Steiger and Jager, 1977), yielded ages of 88.4 way 1. The largest clasts in the conglomerate there appear and 89.6 Ma. Fission-track analyses of two samples (Naeser to be derived from the granitic rock of Montara Moun­ and Ross, 1976) yielded ages of 81.7 and 84.1 Ma on tain, but other clasts are derived from the Franciscan sphene and 23.1 Ma on apatite. According to Compton Complex. On San Pedro Mountain, pebble to cobble (1966), Evernden and Kistler (1970), and Naeser and conglomerate in contact with granitic rock is believed Ross (1976) the resultant ages do not represent time of to represent the basal part of the upper part (unit Tsu) intrusion of the pluton but the time of subsequent (Morgan, 1981b). The conglomerate section appears deformational or heating events. to thin northward from about 300 ft on San Pedro Mountain to about 4 ft (Darrow, 1963) at Point San Pedro. Sheared and brecciated rocks along the contact between conglom­

QUARTZ erate and underlying granitic rock indicate that much­ if not all-of this contact is a fault. Above the conglomerate is a sequence of turbidite sandstone and shale that contains some carbonate beds. Most of the carbonate beds are composed of detrital grains, but some beds are composed of secondary carbonate replacing feldspar and quartz grains (Morgan, 1981a, b; Nilsen and Yount, 1981). Locally the carbonate beds have an aggregate thickness of about 10 ft. No carbonate beds are known from the lower part (unit Tsl). Morgan (1981a, b) and Nilsen and Yount (1981) provide details on the petrology, paleontology, and sedimentology of this sequence of Paleocene rocks. In the sea cliff at Devils Slide, slope failures in the sandstone and shale turbidite deposits cause periodic closure of Highway 1. EOCENE(?) ROCKS At the south edge of the map area, 1 mi west of Carlmont High School, a small patch of unnamed sandstone (Ta) rests on graywacke of the Franciscan Complex. This Figure 3. Modal analyses of igneous rocks of Montara unit was mapped by McLaughlin (1969) as unnamed Mountain classified after Streckeisen (1973). A, aplite; Eocene(?) arenite, an outlier of lithologically similar rocks P, pegmatite; F, felsic rocks; +, Ross (1972); o, this present a mile to the southeast. Although these rocks study. are coarser-grained, they may possibly be the temporal equivalent of Eocene sedimentary rocks 5 mi southeast in Redwood City mapped as the Whiskey Hill Forma­ PALEOCENE ROCKS tion (Beaulieu, 1970, 1971; Pampeyan, 1993). Overlying the granitic basement rock is a sequence of Paleocene turbidite deposits first considered to be part OLIGOCENE AND (OR) MIOCENE ROCKS of the Franciscan Series (Lawson, 1895) and then ten­ Rocks assigned to the Mindego Basalt (unit Tmi) are tatively assigned to the lower Eocene Martinez Forma­ present in a wedge-shaped fault block at the south edge tion (Lawson, 1914). Darrow (1963) divided the sequence of the map area, between granitic rock and the Franciscan into lower (Cretaceous) and upper (Paleocene) units along C::omplex. In the map area this unit is a wacke containing a boulder conglomerate bed at the base of the upper unit, mafic volcanic rock fragments, and it interfingers southward and subsequently the Cretaceous designation was changed with basaltic rocks mapped by Brabb and Pampeyan (1983) to Paleocene on the basis of Ynezian fossils found in as the Mindego Basalt. In roadcuts along Highway 92, upper and lower units (Morgan, 1981a). Studies by Nilsen half a mile south of the map area, this unit is mostly and Yount (1981) and Morgan (1981a, b) refined the weathered amydaloidal basaltic lava with small amounts stratigraphy of the sequence, and Morgan (1981a) di­ of interlayered conglomerate. vided the sequence into three members. On this map the turbidite sequence of sandstone, shale, and conglomerate PLIOCENE ROCKS is divided into lower (unit Tsl) and upper (unit Tsu) parts Sedimentary rocks (unit Tp) exposed in the Seal Cove corresponding to Darrow's (1963) units, along the contact Bluffs and wave-cut bench from Moss Beach south to between Morgan's (1981a) lowest and middle members. Pillar Point (fig. 2) were mapped as the Merced Forma-

6 tion by Lawson (1914) and later reassigned to the Purisima PLEISTOCENE ROCKS AND DEPOSITS Formation by Glen (1959). Glen's correlation was based on lithologic and faunal similarities between his Pillar Point The Colma Formation (unit Oc) underlies much of "Merced" Formation and the type Purisima (Haehl and the historic alluvial plain southeast from San Bruno, at Arnold, 1904), about 9 mi to the southeast. In the map least as far south as Burlingame and possibly to San Mateo. area the bulk of this unit is fine grained and lies south­ Natural exposures are uncommon in most of the area west of the main trace of the Seal Cove Fault where neither but in a few exposures bedding appears to dip less tha~ its base nor its top are exposed. Northeast of the fault, 10°; a few dips as steep as 35° are probably on foreset however, in the east limb of the Moss Beach syncline, beds, which were better exposed in the San Bruno area the basal part of the unit is exposed and consists of boulder­ north of this map area (Bonilla, 1971a). The Colma to pebble-conglomerate beds resting nonconformably on Formation is overridden by rocks of the Franciscan Complex granitic rock. The northernmost exposure of Purisima and Merced Formation in the Serra Fault Zone but appears beds is in the sea cliff 0.5 mi south of Point Montara no more deformed there than in scarce exposures else­ lighthouse where a northeast-striking vertical fault separates where. In places the main criteria for separating the sandstone from granitic rock. Glen's (1959) correlations, Merced from the Colma was the steeper bedding planes along with later information on sense of movement in in the Merced. I saw no fossils in the Colma Forma­ the Seal Cove-San Gregorio Fault Zone, indicate the tion, but in 1962 M.G. Bonilla collected a sea cow rib Purisima of the Seal Cove Bluffs has been offset later­ and other bone fragments from a locality in San Bruno ally northwest an unknown distance from rocks of the about 300 ft east of the trace of the Serra Fault at what type locality of the Purisima Formation across the Seal is now the intersection of Jenevein Avenue and Inter­ Cove-San Gregorio Fault. state 280, a locality destroyed during construction of In­ Glen's (1959, p. 164) study dated the Purisima terstate 280. At that time he questionably assigned the Formation in this area as " ... middle or perhaps even early fossil-bearing beds to the Merced Formation. C.A. Pliocene ... ". Cummings and others (1962) correlated Repenning (written commun., 1989) believes these bones these rocks with the (lower and upper Pliocene) Pomponio indicate a " ... Clarendonian (8-12 Ma) or Hemphillian ... " Member of the Purisima Formation, and Addicott (1969) (middle to late Miocene) age, an age range much older showed them as early Pliocene in age. Marine fauna than that of the fossiliferous upper Pleistocene Colma of the type Purisima indicate the unit is early to late Pliocene Formation or upper Pliocene Merced Formation in this in age (Cummings and others, 1962; Addicott, 1969). area. In the late 1920's a creek-bank exposure of 5 Vertebrate fossils were found in the Purisima Formation to 10 ft of blue-gray sand containing lignitic seams, logs, on the wave-cut bench at Moss Beach in 1970 by Evelina cones, and seeds, overlain by 15 to 20 ft of brownish­ Dunton of San Mateo and between Moss Beach and Seal yellow pebbly sand, was found in San Bruno. This locality, Cove in 1965 by Richard Pefley of Montara. Dunton surrounded by deposits of the Colma Formation, is very found a walrus radius indicating a probably early Hemphillian close to Bonilla's vertebrate locality, and the plant fos­ age (about 7 Ma), and Pefley found a porpoise skull sils were determined to be Pleistocene in age (Potbury, indicating a Hemphillian age (7 -5 Ma) according to C.A. 1932). It is likely that the sea cow bones were reworked Repenning (written commun., 1989). from older deposits or that they represent a block of older deposits in the Serra Fault Zone. Exposures of the Merced Formation (unit Tme) are continuous southeastward from its type section in the Poorly indurated and poorly sorted alluvial depos­ San Francisco South quadrangle (Lawson, 1914; Bonilla, its assigned to the Santa Clara Formation (unit Os) are 1971a) to the vicinity of Burlingame and discontinuous present west of the 1906 trace of the San Andreas Fault as far south as Hillsdale Boulevard in San Mateo-both at the south edge of the map area. A smaller body occurs in and east of the San Andreas Fault Zone. Sandstones as a fault sliver east of the 1906 trace half a mile northwest of the Merced Formation have a distinctive texture and of Crystal Springs Dam. Both bodies have smiliar yellowish color in natural outcrop and are not usually compositions with the exception of the presence of confused with weathered Franciscan graywacke or strata glaucophane schist boulders in the northern body. of the Colma Formation. The lower contact with Franciscan Cummings (1968) divided the Santa Clara Formation into rocks is exposed in several places as is the upper con­ eight lithofacies in his regional study and showed his Crystal tact with the overlying Colma Formation. A complete Springs facies I at the south edge of the San Mateo section, however, was not recognized in the map area, quadrangle (Cummings, 1972). He distinguished his Crystal nor were marker beds with which a composite section Springs facies II from his facies I on the basis of exotic could be constructed. Faulted and folded beds east of rock types in an otherwise lithologically similar unit, and San Andreas Lake suggest the Merced Formation may he deduced that the Franciscan source for these depos­ be as much as 500 ft thick in the map area. Molluscan its was east of the San Andreas Fault. The fault sliver fossils common in the map area indicate a late Pliocene of the Santa Clara Formation, though not shown by age. At its type locality, about 4 mi north of this map Cummings (1968, 1972), may be part of his Crystal Springs area, the Merced is more than 5,000 ft thick and con­ facies II displaced from the main mass of facies U strata tains Pleistocene fossils in its upper part (Hall, 1966) along a separate strand of the San Andreas Fault. as well as an ash bed correlated with the 400,000-yr­ The Santa Clara Formation is generally considered old Rockland ash bed (Sarna-Wojcicki and others, 1985). to be late Pliocene and early Pleistocene in age in its The Pleistocene part of the Merced was not recognized type area at Saratoga, about 22 mi southeast, on the in the map area. basis of plant and animal fossils found there (Sorg and

7 McLaughlin, 1975; Adam and others, 1983). In the Palo Deposits on the second or Montara (Miramar) ter­ Alto 7-1/2' quadrangle, about 10 mi southeast of the race are well exposed along Montara Beach and high­ map area, the Santa Clara Formation rests on fossilif­ way cuts and sea cliffs in Montara. The Montara deposits erous upper Pliocene strata of the Merced(?) Formation are predominantly derived from the underlying granitic (Pampeyan, 1970, 1993) and is thought to be late Pliocene rock, and the younger Half Moon Bay deposits are mainly and early Pleistocene in age (Vanderhurst and others, reworked Montara deposits (Jack, 1969). At the south 1982). Near Woodside, about 6 mi southeast of the map end of Montara Beach a down-faulted remnant of still area the Santa Clara Formation contains an ash bed older terrace deposits is exposed-part of the San Vicente identified by Sarna-Wojcicki and others (1985) as the terrace of Jack (1969). Franciscan rock-types, especially 400,000-yr-old Rockland ash bed. Cummings (1972) well-rounded chert pebbles, are common in these older shows the Crystal Springs facies I and II as overlying his deposits, isolated remnants of which are recognized at Woodside facies in which the ash bed occurs, implying 550-600 ft above sea level north of Princeton (Jack, 1969) that the Crystal Springs facies I and II are late Pleistocene and 500-725 ft above sea level northeast of Princeton. in age. In the type section of the Merced Formation On this geologic map, all the coastal marine terrace deposits the 400,000-yr-old ash occurs in beds assigned to the are shown as a single unit (Omt). Pleistocene upper member of the formation (Glen, 1959; Marine invertebrate fossils from the Half Moon Bay Hall, 1966). It appears, therefore, that the Santa Clara terrace deposits have been dated by amino acid race­ Formation transgresses the Pliocene-Pleistocene boundary mization methods at 80,000 to 85,000 yr B.P. (Kennedy northward along the San Francisco Peninsula and pos­ and others, 1982). Local and regional studies suggest sibly intertongues with the Merced Formation, its tem­ that the Montara (Miramar) terrace deposits may be about poral marine equivalent (Addicott, 1969). 325,000 years old (K.R. Lajoie, oral commun., 1991) Several levels of marine terraces or wave-cut benches and the highest deposits considerably older. C.A. are visible along the Pacific Coast. Smith (1960) sum­ Repenning (written commun., 1989) reports that in 1980 marized information from early coastal terrace studies Oliver Espey of Montara found Smilodon (saber-tooth and within the map area grouped the terraces into Half tiger) and Mammuthus (mammoth) bones on Montara Moon Bay and Mussel Rock sequences. In addition, he Beach 0. 3 mi south of Martini Creek. This locality, at described a Colma terrace sequence on the east side of about mean sea level and often covered by beach sand, the San Francisco Peninsula. The Half Moon Bay and may be in a remnant of the Montara terrace. Accord­ Mussel Rock sequences are equivalent, but their conti­ ing to Repenning the fossils are probably less than 400,000 nuity along the coast is interrupted by Montara Moun­ years old. Discoveries of mammoth bones in San Mateo, tain. According to Smith (1960) the Colma terrace Millbrae, and near Half Moon Bay and bison bones in sequence, which extends from the vicinity of Colma to Pilarcitos Creek previously were assigned to the Pleis­ Belmont, was formed contemporaneously with the Half tocene by Hay (1927). Specific details on the first three Moon Bay-Mussel Rock sequence. A detailed study of localities are not given; the fourth locality is given as about the marine terraces between Montara Beach and Princeton 600 ft upstream from Pilarcitos Lake dam, at a depth by Jack (1969) refined the stratigraphy and structure of of 21 ft, where blue clay was being dug for use in the the coastal terrace sequence. Along the Pacific coast core of the dam (Hay, 1927, p. 121-122). The San the first emergent terrace, Half Moon Bay-Mussel Rock Vicente terrace deposits have not been dated. of Smith (1960), and second terrace, Miramar of Smith Pleistocene coarse-grained fan deposits (unit Ooa) (1960) or Montara of Jack (1969), are well developed are found along the foothills between San Mateo and and easily recognizable; on the San Francisco Bay side San Carlos sloping gently northeast and locally extend­ of the peninsula, however, the Colma terrace sequence ing headward into drainages cutting through the foot­ has been obliterated or obscured by urban development. hills. This unit has been mapped around the edges of Other probable wave-cut surfaces exist as high as 900 San Francisco Bay (Helley and Lajoie, 1979). In the ft above sea level on the southwest-facing slopes of Montara Palo Alto (Pampeyan 1993) and Mountain View (Helley Mountain, and sub-horizontal erosion surfaces at 1, 000- and others, 1972) quadrangles the unit contains a fine­ 1,100, 1,300, and 1,600 ft above sea level northeast grained fossiliferous stratum at its base. Fauna and flora and east of El Granada may represent still older marine from this stratum are about 20,000 years old and are terraces. Marine terrace deposits are continuously present representative of a cooler climate. This fossiliferous stratum along the coast from south of El Granada to Montara was not seen in this map area. Beach and discontinuously north from Calera Valley. Deposits on the lowest (youngest) or Half Moon Bay terrace extend from well south of El Granada to Point Montara HOLOCENE DEPOSITS where they are faulted against older deposits on the Montara Holocene deposits in the map area are widely dis­ terrace (Jack, 1969). The Seal Cove Bluffs, west of Half tributed on the flatlands and also are present locally in Moon Bay Airport, are capped with deposits of the lowest areas of high relief and along the coast. Urban devel­ terrace, uplifted as much as 148 ft along the Seal Cove opments on the gently sloping flatlands have modified Fault (Kennedy and others, 1982). Deposits on the Mussel or obscured these deposits, but studies of old topographic Rock terrace are poorly exposed around Laguna Salada maps and data from wells drilled for water or engineer­ and in Calera Valley and may also exist in San Pedro ing purposes enabled Helley and Lajoie (1979) to iden­ Valley beneath Holocene beach deposits and artificial fill. tify and classify the deposits and have provided some

8 of the information shown on this map. The historic of the map area. It was mapped where believed to ex­ landward extent of bay mud (Om), little of which remains ceed 5 ft in thickness. It is thickest west of the San Andreas exposed, is based on personal observation and U.S. Coast Fault Zone on Franciscan rocks and east of the fault zone Survey (1853a, b; 1854; 1857a, b; 1868) topographic on Franciscan rocks in San Mateo, thinning to zero maps of the margins of the San Francisco Peninsula. Water­ northwest and southeast. The distribution of this unit saturated deposits of bay mud are considered to be highly may be related to slope orientation, slope angle, and susceptible to liquefaction during earthquakes (Y oud and vegetative cover as the mantle appears to be thickest Perkins, 1987) and during the October 19, 1989, Lorna under dense tree and brush cover on moderate to steep Prieta event, sand boils-evidence of liquefaction-formed slopes in rural and undeveloped areas. The distribution along the bay margin 1.3 mi west of Little Coyote Point of small and large landslides show that this unit is sus­ (R.D. Brown, Jr., oral commun., 1989). No similar ef­ ceptible to slope failure, especially when water-saturated fects were reported elsewhere in the map area. (Bonilla, 1960). Deposits created by various types of landslides are Artificial fill in the map area consists of various natural classified as older (unit Ool) and younger (unit Oyl) on and man-made materials emplaced by a variety of methods, the basis of activity, the older ones showing no evidence and where recognizable was mapped as unit Of1• In the of movement in the past several decades (for example, mid-1940's at least some of the fill used in the construction distress of older man-made structures). Within the map of San Francisco International Airport was trucked from area the distribution of older and younger landslide deposits quarries in the Franciscan Complex and Merced Formation shows that sheared rock (unit fsr), slope wash, ravine at the west end of Helen Drive in Millbrae, and in the fill, and colluvium (unit Osr), and weathered granitic rock late 1960's and early 1970's hydraulically-placed fill dredged are especially susceptible to slope failure (Brabb and from adjacent marsh lands and sloughs was placed on Pampeyan, 1972; Brabb and others, 1972; Ellen and Brewer Island and vicinity, the site of Foster City. Some Wieczorek, 1988). In addition, slopes underlain by the of the oldest fills are along or near the historic bay margin Purisima Formation (unit Tp) and upper part of the as levees to create salt evaporation ponds, and some of Paleocene sandstone, shale, and conglomerate (unit Tsu) the youngest are sanitary landfills. Natural phenomena, are prone to failure where oversteepened by wave action residential, commercial, and highway construction con­ or faulting. The large landslides at Seal Cove were studied tinue apace to alter the original topography of the area by Leighton and Associates (1971) and those north of so the extent of artificial fill shown represents only a brief Devils Slide by Beeston and Gamble (1980). Many large moment in time. Early maps (U.S. Coast Survey, 1853a, slides have occurred along Pilarcitos and San Mateo Creeks 1869) show that as late as 1869 the mouth of San Pedro where canyons cut in fault zones have steep, northeast­ Valley contained a lagoon on the submerged Mussel Rock­ facing, colluvium-covered walls and the soil-moisture content Half Moon Bay terrace. By 1892 (U.S. Geological Survey, remains relatively high. Numerous shallow debris flows 1899) the lagoon was replaced by two lakes, the larg­ on Montara Mountain, occurring during or following est named Lake Mathilde, which by 1914 (U.S. Geological prolonged periods of precipitation, have little economic Survey, 1915) had disappeared, probably by natural filling consequence, but in some areas, for example north of with beach and dune deposits, and subsequently this area San Pedro Valley, shallow debris flows in colluvium triggered reached its present state by the use of imported artifi­ by heavy rains in January of 1982 (Howard and oth­ cial fill. Similarly Laguna Salada must have had access ers, 1988) caused deaths and significant property dam­ to the sea and was a natural salt evaporating pond where age. For an analysis of slope stability in the map area early dwellers came to gather salt. Most of the modern under normal and seismic loading conditions, see Brabb fills are engineered while many of the old fills were random and others (1972) and Wieczorek and others (1985). mixtures of rock, soil, and waste materials. Pilarcitos Beach deposits (unit Ob) are present along the coast (1864-66), San Andreas (1868-70), and Upper Crystal and at a few places along the bay margin. The depos­ Springs (1875-77) dams are early examples of engineered, its along the coast are clean siliceous sands derived mainly clay puddle-core, embankments that withstood the 1906 from granitic rock of Montara Mountain, whereas the San Francisco earthquake, the latter two dams being in deposits along the bay are dirty sands derived locally from the San Andreas Fault Zone. Clay for the core of Pilarcitos reworked artifical fill materials. Construction of a breakwater dam came from the floor of Pilarcitos valley, probably in 1959-61 to protect Pillar Point Harbor has disturbed from older alluvium (Hay, 1927, p. 121-122); clay for the transport of sand by longshore currents causing accretion the core of San Andreas dam probably came from similar of beach deposits within the breakwater and erosion of deposits in San Andreas valley. beach and terrace deposits outside the breakwater (Tinsley, In parts of Burlingame, Millbrae, San Bruno, and 1972). At Laguna Salada and at Pacifica beach north­ the Linda Mar district of Pacifica a comparison of 1949 east of Shelter Cove, dunes composed of windblown beach and 1956 editions of the Montara Mountain topographic sand are present along the back edge of the beaches. quadrangle showed areas (not readily discernable during Other areas of dune sand and beach deposits along the field mapping) where rugged topography had been coast are too small to show at map scale. smoothed out. These areas, shown as unit Of2, are believed A mantle of debris referred to as slope wash, ra­ to have been filled during grading operations. In ad­ vine fill, and colluvium (unit Osr), derived from under­ dition there are parts of Linda Mar developed after 1956 lying weathered bedrock units, is present throughout most where the slopes have been steepened by grading; in

9 these areas the geology was mapped on post-1968 aerial a vertical west-side-down fault. Subsequently Lawson (1914) photographs and projected onto the pre-existing topog­ described the Pilarcitos Fault as a northeast-dipping high­ raphy. angle reverse fault, a view shared by Darrow (1963). Younger alluvium (unit Oya) was mapped in a few Wakabayshi and Moores (1988) believe the Pilarcitos Fault drainages where modern runoff carries and moves rock extends only from near Highway 92 northwest to the debris. Some of the areas of coarse-grained alluvium coast as a northeast-dipping thrust-part of a suture between (unit Oac) and slope wash, ravine fill, and colluvium (unit subducted granitic basement rocks and overthrust Franciscan Osr) include strips of younger alluvium too narrow to show. rocks, and that south of Highway 92 several high-angle Most of the drainage channels in urbanized areas have faults are overprinted and obscure the thrust. Smith's been lined or enclosed thereby eliminating the accumu­ (1960) analysis of geomorphic evidence indicates that lation of much of younger alluvium. the Pilarcitos Fault is a right-lateral strike-slip fault in­ termittently active until late Pleistocene time. Dibblee 's Two areas on the map are designated as undivided (1966a) comparison of Tertiary formations along the sedimentary deposits (unit OTs). These areas may in­ Pilarcitos and San Andreas Faults indicates that the Pilarcitos clude various Tertiary to Holocene units, but grading, and San Francisco Bay blocks moved as a unit, " ... the landscaping, and lack of access or urban development main shift being along the Pilarcitos Fault prior to deposition precluded assigment of the earth materials to a specific of the Purisima Formation, and then along the San Andreas map unit. The smaller area is the site of the Green Hills Fault since that time". In summary, the evidence strongly Country Club and probably is underlain by the Merced indicates the Pilarcitos Fault is an ancestral trace of the and Colma Formations and coarse-grained alluvium. The San Andreas Fault with about 20 mi of right slip (Dibblee, larger area probably consists mostly of older alluvium and 1966b); the fault probably also has a vertical compo­ colluvium but may contain some Colma deposits and nent of slip with the northeast side up, possibly a coun­ unnamed Tertiary and Quaternary coarse clastic depos­ terpart of the Serra Fault and other branching reverse its (Lajoie and others, 1974). faults along the northeast side of the San Andreas Fault. The methods of study, characteristics and interre­ The Pilarcitos Fault is not considered to be potentially lations of the Pleistocene and Holocene deposits, and active (California Division of Mines and Geology, 1982) their importance to comprehensive planning are discussed but may be seismically active on the basis of clusters of in detail in Helley and others (1979). Atwater and others seismic activity near Portola Valley, 10 mi southeast of (1977) describe the late Quaternary history of San Francisco the map area, and near Montara Mountain (Brabb and Bay. Olson, 1986). The San Andreas Fault cuts diagonally through the STRUCTURE map area entirely in watershed lands of the City of San The map area is cut by three major faults, the Pilarcitos, Francisco. The fault was formally named by Lawson (1908, San Andreas, and Seal Cove (fig. 1), and the Pilarcitos p. 2) after the San Andreas valley west of Millbrae, though Fault marks the boundary betweer, two different types he previously mentioned the " ... San Andreas Fault..." of basement rocks, the Franciscan Complex on the east (Lawson, 1895, p. 441) after commenting on " ... a and granitic rock on the west. The named faults typi­ remarkably straight fault which has conditioned the San cally are zones of faulting ranging in width from a few Andreas and Spring Valley, one of the most notewor­ feet to 500 ft or more. The lines shown on the map thy features of the topography." (Lawson, 1895, p. 439). represent the most prominent-and most recently active­ The San Andreas Fault is the boundary between the Pacific traces of faults in the respective zones. and North American crustal plates which are sliding past The trace of the Pilarcitos Fault, from its point of each other at a rate of about 1.38 to 1. 97 in. (35 to divergence with the San Andreas Fault near the San Mateo­ 50 mm) per year (Argus and Gordon, 1990). Through­ Santa Cruz County boundary to the head of San Pedro out much of California this boundary separates granitic Valley at Whiting Ridge, is not deflected as it crosses basement rocks from Franciscan basement rocks, the ridges and ravines indicating a vertical or very steeply Salinian and Northern Franciscan areas of Reed (1933), dipping fault plane not unlike the vertically dipping zone but in this area, as noted above, the Pilarcitos Fault forms of sheared Paleocene rocks exposed in the east abut­ the basement boundary. ment of the Pilarcitos dam. The remainder of the fault In this area the San Andreas Fault is a zone of faulting onshore, in part confined to a narrow linear ravine, is with a maximum width of about 500 ft. Within the zone concealed by Quaternary deposits in San Pedro Valley; only the most recently active traces are shown, those offshore the fault continues northwest and merges with thought to mark the 1906 surface rupture (Pampeyan, the San Gregorio Fault which in tum joins the San Andreas 1983). Several large historic earthquakes have occurred Fault off the Golden Gate (McCulloch, 1989, fig. 12). on this fault, the most notable in northern California being Lawson (1895, p. 437) first recognized a significant the April 18, 1906, M 8.2 San Francisco earthquake fault in Pilarcitos canyon along which granitic rocks on whose epicenter is generally located offshore between the southwest side were thrust up against Franciscan rocks, Mussel Rock and Bolinas Bay (about 12 mi northwest and he extended the fault to the coast between Spring of the map area). Within the map area fences, pipe­ Valley and Fifield Ridges to the mouth of Calera Valley lines, roads, and other manmade structures were offset (Lawson, 1908, pl. 15). Branner and others (1909) as much as 11 ft in that event (Schussler, 1906; Lawson, extended the fault 18 mi southeast of the map area as 1908; Pampeyan, 1983), and possibly more. The historical

10 records for earlier earthquakes in this area are less accurate Cove segment and the main part of the San Gregorio and leave room for speculation on location of epicen­ Fault lying a few miles offshore and parallel to the Seal ter, magnitude, exact date, and length of surface rup­ Cove Fault (McCulloch, 1989). At Moss Beach an excellent ture for major events. In 1906 the surface rupture sea-cliff exposure of the Seal Cove Fault showing 80,000 resembled one or more plowed furrows-except that the to 85,000-yr-old marine terrace deposits against lower sod was not turned over-some in straight lines and others Pliocene Purisima mudstone was covered with riprap in in en echelon patterns (Lawson, 1908, pl. 61). The the 1980's to retard erosion of the cliff. Numerous faults movement was mainly right-lateral strike-slip, but small and lineaments parallel to the Seal Cove Fault are vis­ components of vertical displacement were reported in ible in the sea cliff near Moss Beach, in the wave-cut a few places (Lawrence, 1924; Pampeyan, 1983, 1986). bench along the Seal Cove bluffs, and in marine terrace Within the map area much of the 1906 surface rupture deposits west and north of Half Moon Bay airport. The was under water, but a fault plane was exposed at one Seal Cove bluffs themselves are cut by faults, one of which locality believed to be near the northwest end of Lower juxtaposes the Purisima Formation and granitic rock under Crystal Springs Lake (Lawson, 1908, pl. 62A; Pampeyan, a cover of terrace deposits (Leighton and Assoc., 1971). 1983); otherwise the 1906 surface rupture zone has been Some of these faults coincide with lineaments visible on exposed only during drought years of 1924 and 1931 aerial photographs, the youngest and most significant or on rare occasions when lake levels were lowered for example, referred to here as the Denniston Creek Fault, construction purposes (Pampeyan, 1975, 1983). Branching cuts Holocene alluvium (Lajoie and others, 1972; Weber and secondary faults also were reported in 1906, for and Lajoie, 1980). All of these faults and lineaments example near Crystal Springs Dam " ... cracks emerged are presumed to be in the San Gregorio-Hosgri Fault Zone. from the lake and ran northward up on the hills for several The amount of right slip on the Seal Cove Fault is not hundred yards, breaking fences where they crost." (Lawson, known but Glen's (1959) correlation of Pliocene rocks 1908, p. 93), and between San Andreas Lake and Mussel in the Seal Cove bluffs with the type Purisima Forma­ Rock to the north " ... there were, in general, furrows on tion at Purisima Creek, about 9 mi south of the map either side of the main fault, at various distances up to area, suggests at least 9 mi of lateral offset; the vertical 200 feet. Some of these were persistent for consider­ component of slip on the Seal Cove Fault is at least 148 able distances." (Lawson, 1908, p. 95). ft adjacent to the Half Moon Bay Airport on the basis Following the 1906 earthquake the State Earthquake of offset terrace deposits (Kennedy and others, 1982). Investigation Commission established a quadrilateral of The Seal Cove and Denniston Creek Faults are consid­ monuments across the fault rupture, about 2 mi north­ ered to be potentially active and have been zoned ac­ west of Crystal Springs Dam (Lawson, 1908, p. 156), cordingly for Special Studies (California Division of Mines for the purpose of making exact measurements of any and Geology, 1982). Theodolite measurements across future movements. The quadrilateral was resurveyed in the Seal Cove Fault at a site west of Princeton showed 1947, 1957, and 1963 and according to Parkin (1965, left slip of 0.156 in. between November 1979 and February p. 5) the changes were " ... practically negligible." and did 1980 and then right slip of 0.256 in. between Febru­ not establish a trend. In 1973 I found the northwest ary 1980 and April 1981, but these data did not show and southwest monuments still in place, the northeast a trend (Galehouse, 1981). monument had been disturbed during construction of a A zone of southwest-dipping reverse faults named pipeline, and I did not find the southeast monument. the Serra Fault Zone is east of and subparallel to the Measurements of displacement across the most recently San Andreas Fault from San Bruno to Hillsborough. active strand of the San Andreas Fault in Woodside, about Faulting in this zone was first noted in 1955 in the Montara 3.5 mi southeast of the map area, demonstrate horizontal Mountain quadrangle by M.G. Bonilla and about 1957 deformation at varying rates which average out to a tectonic named on his geologic map of the San Francisco South slip rate of 0.09 in. (2.2 mm)/yr (Harsh, 1977). Long­ quadrangle (1965, 1971a). In 1955 M.G. Bonilla and term geodimeter measurements across the map area show C.R. Appledorn examined and photographed an artifi­ no aseismic slip but about 0.4 in. (10 mm}/yr of strain cial exposure about 1/2 mi east of Crestmoor High School, build-up across the San Andreas Fault Zone (Bennett, at the east edge of the present North Park. They 1980), suggesting the total plate motion is distributed determined that one of the shallow west-dipping faults over a wider zone of coastal California. The San Andreas exposed there displaced beds of the Merced Formation, Fault is tectonically active and is zoned for Special Studies "old alluvium", and modern soil (M.G. Bonilla, written (California Division of Mines and Geology, 1974, 1982). commun., 1992). Smith (1960, fig. 75) photographed The Seal Cove Fault is a part of the San Gregorio­ a few strands of the fault, which were exposed during Hosgri Fault Zone, a mostly offshore zone of faulting grading operations in west Millbrae, and plotted their extending about 270 mi from the vicinity of Bolinas Bay positions on his map. This zone of faulting approximately south to near Point Sal (Graham and Dickinson, 1978; coincides with part of the Foothill Thrust, a line drawn McCulloch, 1989). The onshore parts of this zone have along the east base of the Santa Cruz Mountains by Willis been mapped in segments by Weber and Lajoie (1980) (1938, fig. 1). Apparently Willis believed the mountain­ who described geomorphic evidence for recency of flatland boundary should be a reverse fault but he pre­ movements on each segment. One of these segments, sented no evidence to accompany his sketch map. A the Seal Cove Fault, lies at the base of a east-facing scarp similar line labelled as the Belmont Fault-possibly based along the west edge of the Half Moon Bay Airport. The on Willis' Foothill Thrust-was shown by G.F. Engle in Seal Cove bluffs are an upthrown block between the Seal a memorandum report on the Crystal Springs Dam made

11 for the San Francisco Water Department in 1930. Engle's Darrow (1963) and Morgan (1981a) each mapped Fault extends northwest from Laurel Creek, 0.6 mi south­ faults along part of the boundary separating granitic and west of Hillsdale High School, through the intersection sedimentary rocks on San Pedro Mountain. This boundary, of highway 1-280 and the north edge of this map in nearly here referred to as the San Pedro Mountain Fault, extends a straight line as far north as the Chinese Cemetery in southeast from Devils Slide (Beeston and Gamble, 1980) Colma (about 19 mi). In places this line coincides with to Pilarcitos Lake and beyond to join the Pilarcitos Fault. mapped traces in the Serra Fault zone. At the north Over most of its length the fault is poorly exposed, but edge of the map one strand of the Serra Fault dips 20°- where exposed it is marked by a narrow sheared and 400 west and places Franciscan sheared rock (unit fsr) brecciated zone dipping 40°-60° north. At its north­ on the Colma Formation. Elsewhere to the southeast west end, in roadcut and seacliff exposures north of Devils imbricated strands juxtapose Franciscan and Merced rocks Slide, the main fault strand dips gently north under massive in a zone at least 3,000 ft wide. The zone of faulting slope failures in Paleocene sedimentary rocks; near its may be wider and extend farther southeast than shown southeast end the fault is poorly exposed but appears along both the Merced-Colma contact and (possibly) the to branch before merging with the Pilarcitos Fault. Franciscan Complex-Holocene deposits contact. The Serra Additional information on the major faults and Fault is not known to be seismically active and has not seismicity in San Mateo County can be found in Brabb been zoned for Special Studies (California Division of and Olson (1986) and Wesson and others (1975). Mines and Geology, 1982). Compressional deformation resulting from ongoing In Belmont and San Carlos a steeply dipping to vertical, tectonic activity and strain build-up in this part of the northwest-striking fault cuts through the hills and passes Santa Cruz Mountains is evidenced by folding in depos­ under alluvium at either end. This fault referred to as its as young as late Pleistocene. In the Pigeon Point the Belmont Hill Fault (Pampeyan, 1981b), is in Franciscan and La Honda blocks the youngest emergent marine terrace chert and graywacke, both of which have been locally deposits (80,000-85,000 yr B.P.) and the underlying Half altered to clay minerals along the fault. On Belmont Moon Bay-Mussel Rock terrace are gently warped. Hill some small landslides are in clay-rich rock and soil According to Lajoie and others (1972) the Half Moon on oversteepened slopes along the fault. The Belmont Bay terrace gradually drops in elevation from about 20 Hill Fault probably passes east of an outlier of chert on ft above sea level near Point Montara to 60 ft below sea the west side of El Camino Real near Hillsdale Boule­ level south of Half Moon Bay Airport and rises to 140 vard and may extend through or just east of another outlier ft above sea level10 mi south of El Granada. The equivalent of chert and graywacke in central San Mateo. Half a Mussel Rock terrace also is warped and falls from over mile east of the Belmont Hill Fault, a fault referred to 200 ft above sea level 2 mi north of the map area to as the Angelo Fault cuts Franciscan conglomerate, below sea level at Laguna Salada (Schlocker and Bonilla, graywacke, and greenstone, the latter being altered to 1962; Bonilla and Schlocker, 1966). Calera and San clay minerals near the fault. The fault is named for U.S. Pedro Valleys are drowned parts of this terrace. Mod­ Coast Survey (1853b) triangulation station ANGELO which erate folding in the lower Pliocene Purisima Formation at one time occupied the summit of a small hill at this is well exposed in the wave-cut bench between Moss Beach site. The strikes of the Belmont Hill Fault, Angelo Fault, and the south edge of the map, for example, the Moss and an unnamed fault in the basement-rock outlier in Beach Syncline and the Whalemen 's Harbor Anticline. central San Mateo are parallel and project northwestward Lawson (1914, p. 16) speculated that these beds, which toward a subsurface projection of the San Bruno Fault rest nonconformably on granitic rocks, were " ... crowded (Lawson, 1914; Bonilla, 1971a). The existence of a fault horizontally ... " and folded without deforming the underlying or fault zone under the flatlands is suggested by a trough erosional surface. The Paleocene rocks at Devils Slide of Pliocene(?) to Holocene deposits between basement have undergone even more compressional deformation highs at Coyote Point, Little Coyote Point, and central and are isoclinally folded and locally overturned. Smith San Mateo. Water-well logs show an apparent step down (1960, p. 130-135) found no evidence to indicate warping of basement between these basement highs in a trend of his Colma sequence of terraces, but moderately dip­ that extends southeast towards Redwood City (Hensolt ping upper Pliocene Merced strata and gently dipping and Brabb, 1990). Pleistocene Colma strata suggest deformation may be pro­ Other prominent faults or zones of faulting in the ceeding at a similar rate in the San Francisco Bay block. map area are recognized but little is known of them because Franciscan rocks in the San Francisco Bay block are of their inactivity, inaccessibility, and(or) lack of expo­ folded, but no areal or regional trend was obvious except sure. The San Mateo Fault, west of the San Andreas perhaps for an apparent eastward dip of the sheared rock­ Fault, follows a structurally controlled drainage but little graywacke contact between San Mateo and San Carlos. more is known about it. The north arm of Pilarcitos In the Pilarcitos block Lawson (1914) showed the main Lake also is in a fault-controlled ravine, and the north­ northwest-trending ridges as open synclines, separated west-striking serpentinite bodies on Fifield Ridge follow by faults, with greenstone or limestone in their cores. a fault which in part separates greenstone and graywacke That may be an accurate representation of the overall from sheared rock (unit fsr). As noted above, faults cutting structural setting, but in places, for example on Cahill the sheared rock unit locally consist of narrow steep zones Ridge, I found no clues as to how the outcrops on the of shearing or gouge in rocks showing flat-lying or gently southwest side of the ridge were related to outcrops along dipping shear foliation.

12 Lower Crystal Springs Reservoir on the northeast side turn of the century evaporation ponds were created in of the ridge. the vicinity of Belmont Slough by building levees on the The geomorphology of the map area has been marshlands. These ponds were used in the salt-making discussed by Lawson (1895, 1914) and Smith (1960), process for about 60 years before production here was as parts of studies covering larger parts of the San Francisco phased out and moved to the east edge of the bay. Bay region. Large-scale urbanization of the map area According to Ver Planck (1958) a Leslie Salt Refinin0 beginning in the 1950's has changed much of the land Company was in operation in San Mateo in 1901, befor~: surface so some of the critical geomorphic evidence no the main operations began in Redwood City in 1908. longer exists or is not easily decipherable. In the 1960's many of the older evaporation ponds were filled and converted to residential and other commercial uses. GEOPHYSICAL STUDIES Another nonmetallic non-rock commodity of the map An aeromagnetic study of the San Francisco Bay area was oyster (Ostrea /urida) shells dredged from the region shows three linear positive anomalies in the map bay for use in making Portland cement. In 1924 the area that are interpreted to represent steeply dipping tabular Pacific Portland Cement company began an operation bodies of serpentinite in zones of crustal weakness (Brabb in Redwood City (Bowen, 1957), which continued until and Hanna, 1981). These positive anomalies are named the plant became uneconomic to maintain in the 1970's. (1) San Pedro anomaly, (2) Pulgas Ridge anomaly, and The oyster shells provided lime and the accompanying (3) Redwood City anomaly (fig. 4). The San Pedro anomaly clayey sediment provided alumina, silica, and iron ox­ extends across the map area from the south end of Cahill ide for the process (Bowen and Gray, 1962a, b). Ridge northwest to Rockaway Beach and beyond, subparallel to but just northeast of the Pilarcitos fault. North of San Rock units of the Franciscan Complex have been Pedro Valley this anomaly appears to follow the con­ used in various ways. During construction of Crystal Springs tact between sheared Franciscan rock (unit fsr) and relatively concrete arch dam (1887-1890), graywacke, hand-quarried unbroken bodies of greenstone and graywacke. The Pulgas from a tectonic inclusion 0.6 mi northwest of the dam Ridge anomaly trends northwest along Pulgas and Buri site, was used as aggregate in the dam (Watts, 1888; Buri Ridges, possibly in response to a steeply dipping Spring Valley Water Works, Photo Album, 1866-1904, serpentinite source from which the exposed gently dip­ photo no. 83, p. 34 and photo on. p. 38, both photos ping serpentinite sheet originated. The northwest end c. 1887, San Francisco Public Utilities Commission photos of the Redwood City anomaly (the Redwood City Fault X-7842 (D-1244) and M-1755, respectively). Conglom­ Zone of Brabb and others [1990]) is east of Coyote Point. erate, greenstone, and chert were quarried for use as Figure 4 shows the Redwood City anomaly is divided into fill from a hill locally known as Quarry Hill, the former north and south branches, interpreted to reflect the presence site of U.S. Coast Survey triangulation station ANGELO of a faulted or folded sheet of serpentinite at depth. The in Belmont, and chert from small quarries along on the two branches of the Redwood City anomaly merge and west side of 1-280 has been used for road metal in San extend southeast across the adjacent Redwood Point and Francisco watershed lands. Sheared rock (unit fsr) and Palo Alto quadrangles (Brabb and others 1990; Pampeyan, sandstone from the Merced Formation was quarried in 1993). No magnetic anomalies were noted along the Millbrae in the late 1940's and early 1950's between San Andreas Fault in the map area. A broad negative the present west end of Helen Drive and 1-280 for use anomaly centered over Montara Mountain is believed to as fill to extend the runways of San Francisco airport, indicate reversed magnetic polarity of the granitic plu­ prior to residential development of western Millbrae. The ton (Brabb and Hanna, 1981). Calera Limestone was quarried in and near Rockaway Beach and on Cahill Ridge for use as rip rap, road metal, Gravity studies in the San Francisco Bay region concrete aggregate, and in sugar refining. Hay (1927) (Chapman and Bishop, 1968; Robbins and others, 1982) reported that blue clay from Pilarcitos canyon was used show the map area to be dominated by a positive anomaly for the impervious core of Pilarcitos dam. Clay from centered on the granitic rock of Montara Mountain and an unknown-but presumably local-source also was used adjacent dense Franciscan rocks. This anomaly is elongate for the "puddle core" of San Andreas Lake dam in 1868- subparallel to the San Andreas Fault and slopes north­ 1869 (Pampeyan, 1983). Serpentinite slopes in the east east to a low along the east edge of San Francisco Bay. abutment of San Andreas Lake dam are scarred and may The positive anomaly crosses both the Pilarcitos and San have been a source for some of the embankment material· Andreas Faults obliquely without any distinctive breaks a nearby quarry (in sandstone) postdates construction of to mark these faults. the dam. Specific sites for major sources of embank­ ment material for the Pilarcitos Lake (1984-66), Upper MINERAL RESOURCES Crystal Springs Reservoir (197 5-7 7), and San Andreas To date no economic deposits of metallic minerals Lake (1868-69) dams are unknown but must have been have been found in the map area, and the non-metallic close to the construction sites; embankment material for mineral commodities produced in the map area consist the Pilarcitos dam may also have included granitic rock chiefly of rock products. One exception is salt (NaCI) and Paleocene sedimentary rocks. gathered by early dwellers of the region at Laguna Salada, Tne granitic rock of Montara Mountain (unit Km), which at one time was a natural evaporation pond on in the form of "decomposed granite" from several small the "drowned" Mussel Rock marine terrace. Around the quarries between El Granada and the vicinity of Montara

13 Beach, has been used for fill and base rock in road con­ Surficial unconsolidated deposits-including bay mud, struction. Fresh granitic rock is quarried in Nuff Creek, along with bedrock units, have been widely used for artificial just south of the map area, for various uses where only fill, especially along the edge of San Francisco Bay where minus 1-1/2-in- size material is needed. In open-cut ex­ a historic network of sloughs and intervening marshland posures more than 50 ft deep, pervasive fracturing allows have been covered and developed for residential and the fresh rock to be ripped and loaded without crush­ commercial uses. ing, and only rarely is blasting required.

37' 37' Area of geologic map 30"

...... •. (1"1• "'?- ••• ~··. ~· ~··. • ~··. ~··. (1"1' Q0 ••• ~·. 0 •• -<' •• "'C. •• ':A •••••••• 37' 22' 30" EXPLANATION Crestline of linear positive magnetic anomaly

Boundary of negative magnetic anomaly-Hachures point toward lower valve

Complete Bouguer gravity contour. H, gravity high; L, gravity low

Fault-Dotted where concealed; queried where uncertain

Figure 4. Complete Bouguer gravity contours, aeromagnetic anomalies, and principal faults in the study area and vicinity. Gravity data from Robbins and others (1982); contour interval 2 mGals. Aeromagnetic data from Brabb and Hanna (1981) and V.E. Langenheim and R.F. Sikora (unpub. data, 1991).

14 REFERENCES CITED Bonilla, M.G., 1960, Landslides in the San Francisco South quadrangle, California: U.S. Geological Survey Adam, D.P., Mclaughlin, R.J., Sorg, D.H., Adams, D.B., Open-File Report, 44 p. Forester, R.M., and Repenning, C.A., 1983, An animal- and plant-fossil assemblage from the Santa ___ 1965, Geologic map of the San Francisco South Clara Formation (Pliocene and Pleistocene), Saratoga, quadrangle, California: U.S. Geological Survey Open­ California, in Anderson, D.W., and Rymer, M.J., File Map 65-18, scale 1:20,000. eds., Tectonics and sedimentation along faults of _____ 1971a, Preliminary geologic map of the San the San Andreas system: Los Angeles, Society of Francisco South and part of the Hunters Point quad­ Economic Paleontologists and Mineralogists, Pacific rangle, California: U.S. Geological Survey Miscel­ Section, p. 105-110. laneous Field Studies Map MF-311, scale 1:24,000. 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